Hydraulic actuator for a gas exchange valve

ABSTRACT

A hydraulic actuator for gas exchange valves of internal combustion engines is disclosed, in which the lifting of the gas exchange valve from the valve seat is effected with great force. The opening motion of the gas exchange valve then ensues at reduced force. Upon closure of the gas exchange valve, the gas exchange valve is braked before striking the valve seat, so that operation of the gas exchange valve with little wear and little noise can be achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE02/02791 filed onJul. 30, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a hydraulic actuator for a gas exchange valvefor internal combustion engines.

2. Description of the Prior Art

The opening and closing of the gas exchange valve should be as fast aspossible, in order to minimize flow losses from the gas exchange valveeither when the combustion air is aspirated or upon expulsion of theexhaust gases from the combustion chamber.

The overpressure intermittently prevailing in the combustion chamber ofthe engine presses the gas exchange valve into the valve seat. Becauseof this overpressure, opening the gas exchange valves requires anincreased expenditure of force for lifting the gas exchange valve, inparticular the outlet valve, from the valve seat. Once the gas exchangevalve has lifted from the valve seat, the pressure in the combustionchamber drops sharply, so that the force needed to open the gas exchangevalve is correspondingly less.

Upon closure of the gas exchange valve, it must also be noted that thespeed at which the valve plate of the gas exchange valve strikes thevalve seat should not be excessive. If that speed is too high, unwantednoise and increased wear occur when the valve plate strikes the valveseat.

The object of the invention is to furnish a hydraulic actuator for a gasexchange valve which can exert a strong force at the onset of theopening motion on the gas exchange valve, which enables fast controlmotions of the gas exchange valve, and in which the gas exchange valvestrikes the valve seat at low speed.

According to the invention, this object is attained by a hydraulicactuator for a gas exchange valve of an internal combustion engine,having a cylinder bore, having a piston, and having an annular piston,the piston and the annular piston being guided in the cylinder bore, andthe piston, annular piston and cylinder bore define a first chamber inthe axial direction whose volume increases when the actuator opens thegas exchange valve, and the annular piston and the cylinder bore definea second chamber in the axial direction whose volume decreases when theactuator opens the gas exchange valve, and the piston and the cylinderbore define a third chamber whose volume decreases when the actuatoropens the gas exchange valve, and having a device for limiting thevolumetric decrease of the second chamber.

SUMMARY AND ADVANTAGES OF THE INVENTION

In the hydraulic actuator of the invention, at the onset of the openingmotion of the gas exchange valve, a strong hydraulic force istransmitted by the actuator to the gas exchange valve, so that despitethe contrary pressure on the valve plate of the gas exchange valve fromthe combustion chamber, the gas exchange valve can be lifted securelyand quickly from the valve seat. As soon as the force needed to actuatethe gas exchange valve has decreased, for instance because there is nolonger any substantial contrary pressure in the combustion chamber, theannular piston is no longer moved onward, and consequently only a lesserhydraulic force is now exerted on the piston of the actuator, and thislesser force is transmitted in turn to the gas exchange valve. With thereduction in the hydraulic force, the energy required to adjust theactuator piston is also reduced, so that the overall energy required forvalve control of the engine drops. Simultaneously with the reduction inthis force, the adjusting speed of the gas exchange valve also varies.Finally, upon closure of the gas exchange valve, braking of the gasexchange valve by the hydraulic actuator of the invention can beachieved before the gas exchange valve strikes the valve seat of theengine. This reduces the wear to the valve seat and gas exchange valveand also lessens the noise produced by the valve control of the engine.

The onset of the braking operation of the gas exchange valve upon itsclosure is moreover independent of production tolerances in the gasexchange valve and of the temperature-caused changes in length thatalways exist in internal combustion engines because of thermalexpansion. With the actuator of the invention, highly stable operationof the engine can therefore be achieved and is affected by neithertemperature expansions nor production tolerances.

In a variant of the invention, it is provided that the piston has aplunge cut; that the annular piston has a stepped center bore with onelarger diameter and one smaller diameter; and that the annular pistoncan be slipped by the larger diameter of the center bore onto thepiston, so that the ratio of the actuating forces of the actuator uponopening of the gas exchange valve and during the remaining adjustingmotion is adjustable in a simple way.

This effect can be further enhanced by providing that the diameters ofthe piston on both sides of the plunge cut are different; and that theannular piston can be slipped onto the larger diameter.

In a further feature of the invention it is provided that the device forlimiting the volumetric reduction of the second chamber is a pressurereservoir that is in communication with the second chamber and that hasa piston; and that the travel of the piston is limitable, so that theannular piston can be arrested in a simple way by hydraulic means. Sincethe pressure reservoir does reach the high temperatures of the gasexchange valve and the cylinder head of the engine, the position inwhich the annular piston is arrested after the gas exchange valve hasopened is independent of the thermal expansions of the gas exchangevalve and of the cylinder head.

Further features of the invention provide that the pressure reservoir isa spring reservoir or a gas reservoir, and/or that the travel of thepiston is limitable by a stop, in particular an adjustable stop, so thatthe actuator of the invention can be adjusted simply.

Further features of the invention provide that the first chamber can bemade to communicate with a pump via a first switching valve; that thesecond chamber can be made to communicate with an oil pump via a secondswitching valve; and that the third chamber is acted upon by the feedpressure of the pump, so that by the actuation of two switching valves,the gas exchange valve can either be opened or closed by the hydraulicactuator of the invention, and the increased force upon liftoff of thegas exchange valve from the valve seat and the slowing down of the gasexchange valve before it strikes the valve seat can be realizedautomatically by the hydraulic actuator of the invention.

Separate triggering of the hydraulic actuator for that purpose isunnecessary. This makes the work of the control unit required fortriggering the actuator easier, and makes the hydraulic actuator of theinvention robust and insensitive to external factors.

The action according to the invention of the actuator is furtherreinforced by the provision that the first chamber and the secondchamber are hydraulically in communication with one another via athrottle, in particular an adjustable throttle, and/or that a checkvalve is provided between the second chamber and the first chamber andblocks the hydraulic communication from the first chamber to the secondchamber. The throttle has a definitive influence on the braking of thegas exchange valve before it strikes the valve seat.

In an advantageous embodiment of the invention, the device for limitingthe volumetric decrease in the second chamber has a shutoff valve whichis in communication with an opening in the second chamber and which inone switching position closes the opening and in its other switchingposition opens it to allow fluid to flow out. With the closure of theshutoff valve, the annular piston is fixed, so that the instant ofclosure of the shutoff valve defines the stroke length of the annularpiston. The instant of onset of the braking action upon closure of thegas exchange valve is in turn dependent on the stroke length of theannular piston; this braking action ensues earlier with a longer strokeof the annular piston and later with a shorter stroke. Thus by means ofthe shutoff valve, the onset of braking can be adjusted independently ofproduction tolerances or material expansions caused by temperaturefluctuations.

In an advantageous embodiment of the invention, the shutoff valve is notused as an additional component unit; instead, its function is allocatedto the second switching valve, which is required anyway to initiate theclosing operation of the gas exchange valve. With the omission of theshutoff valve and by dispensing with the above-described pressurereservoir for the device for limiting the volumetric decrease in thesecond chamber, the construction costs for valve control are reduced.

In an advantageous embodiment of the invention, between the firstchamber and the throttle disposed between the two chambers for varyingthe braking behavior of the actuator piston and thus of the gas exchangevalve, a flow-controlled valve is provided which is embodied such thatit is closable by the fluid flowing to the first chamber. This has theadvantage that in the initial phase of the stroke of the actuatorpiston, in which both switching valves are open, fluid from the firstswitching valve cannot flow directly via the throttle into the hydraulicrelief chamber or oil sump. It is true that if the throttling action ofthe throttle is strong, this flow-controlled valve can be dispensedwith, since only slight quantities of fluid flow out via the throttle;however, if there is a relatively large throttle opening for the sake ofattaining an only slight braking action at the gas exchange valve, thenthe flow-controlled valve is indispensable for blocking off thethrottle, if major leakage is to be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail in the ensuing descriptionof exemplary embodiments, taken in conjunction with the drawings, inwhich:

FIG. 1 is a schematic illustration of a longitudinal section through ahydraulic actuator of the invention, with its hydraulic connection;

FIG. 2, a longitudinal section through the actuator of FIG. 1 in threedifferent positions;

FIGS. 3 and 4, respective fragmentary longitudinal sections through theactuator of FIG. 1 with a variously modified hydraulic connection; and

FIGS. 5 and 6, respective longitudinal sections through aflow-controlled valve of FIG. 4, in the open state (FIG. 5) and in theclosed state (FIG. 6).

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an exemplary embodiment of a hydraulic actuator with ahousing 1 in longitudinal section. The housing 1 has a stepped cylinderbore 3. To simplify production, a sleeve 5 is press-fitted into thehousing 1, and its inner bore defines part of the stepped cylinder bore3. In the region of the sleeve 5, an annular piston 7 and a piston 9 areguided in the cylinder bore 3. In the position of the piston 9 as shownin FIG. 1, the gas exchange valve, not shown, is closed.

The cylinder bore 3, piston 9 and annular piston 7 define a firstchamber 13 in the direction of a longitudinal axis 11 of the piston 9.So that no liquid or fluid can escape between the cylinder bore 3 andthe piston 9, a first sealing ring 15 is disposed on the left-hand end,in terms of FIG. 1, of the first chamber 13.

The piston 9 has a plunge cut 17. The diameters of the piston 9 onopposed sides of the plunge cut 17 are of different sizes. On the sidetoward the sealing ring 15, the piston 9 has a smaller diameter d₁, andon the other end of the plunge cut 17, the piston 9 has a largerdiameter d₂.

The annular piston 7 is disposed between the sleeve 5 and the piston 9.The annular piston 7 is fitted into the cylinder bore 3 in such a waythat on the one hand it is displaceable in the axial direction, and onthe other, a good sealing action is attained between the cylinder bore 3and the annular piston 7. The annular piston 7 has a stepped center bore19, with one smaller diameter d₃ and one larger diameter that is thesame size as d₂. The fit between the annular piston 7 and the largerdiameter d₂ of the piston 9 is likewise selected such that the annularpiston 7 and the piston 9 are movable relative to one another in theaxial direction, yet nevertheless a good sealing action is achieved.

On the right-hand side, in FIG. 1, of the annular piston 7, the cylinderbore 3 and the annular piston 7 define a second chamber 27. In thisregion, the cylinder bore 3 has a diameter d₄, which is equal to theouter diameter of the annular piston 7. The piston 9, on its right-handend in terms of FIG. 1, has a shoulder with the diameter d₅.

On the right-hand end, in FIG. 1, of the cylinder bore 3, the annulargap between the cylinder bore 3 and the piston 9 is bridged by a secondsealing ring 21 and is sealed off from the environment. On this end ofthe piston 9, the shaft 23 of a gas exchange valve, shown only in part,is connected by positive engagement to the piston 9.

Between the shoulder of the piston 9 having the diameter d₅ and thecylinder bore 3 having the diameter d₂, there is a third chamber 25,which is sealed off from the environment by the sealing ring 21. Theannular piston 7, the part of the cylinder bore 3 having the diameterd₄, and the piston 9 define the second chamber 27. The first chamber 13can be made to communicate hydraulically with a pump 31 via a firstswitching valve 29. The first switching valve 29 can be embodied forexample as an electrically actuated magnet valve.

The pump 31 permanently subjects the third chamber 25 to the feedpressure that it generates.

By means of a second switching valve 33, embodied for example as anelectrically actuated magnet valve, a hydraulic communication can beestablished between the second chamber 27 and a relief chamber or oilsump 35. A check valve 39 is disposed in a line 37 that connects thesecond chamber 27 and the second switching valve 33. A hydraulicreservoir 41 is connected between the check valve 39 and the secondchamber 27. The hydraulic reservoir 41 has a piston 43, which movescounter to the force of a spring 45 when the pressure exerted on theface end of the piston 43 remote from the spring 45 is high enough. Thispressure is equal to the pressure in the line 37. The travel of thepiston 43 counter to the force of the spring 45 is limited by a stop 47,which may also be embodied adjustably. Between the first chamber 13 andthe second chamber 27, a hydraulic communication is provided in which anadjustable throttle 49 is disposed.

When the first chamber 13 and the third chamber 25 are acted upon by thefeed pressure of the pump 31, which is the case when the first switchingvalve 29 is open, then various hydraulic forces, which will now bedescribed, act on the piston 9:

The diameter d₄ of the cylinder bore 3, the annular piston 7, and theright-hand side, in FIG. 1, of the plunge cut 17 form a first annularface A₁ with an outer diameter d₄ and an inner diameter d₆, the latterbeing equivalent to the inner diameter of the plunge cut 17. Thepressure of the hydraulic fluid, located in the first chamber 13 andacting on the first annular face A₁, seeks to move the piston 9 to theright. The resultant force is responsible for the opening of the gasexchange valve, not shown.

The shoulder on the right-hand side, in FIG. 1, of the plunge cut 17,which is defined by the diameters d₂ and d₆, will hereinafter also becalled the second annular face A₂.

The hydraulic force exerted on the first annular face A₁ is reduced bythe hydraulic forces acting on a third annular face A₃ and a fourthannular face A₄.

The third annular face A₃ is defined by the shoulder in the piston 9that is formed by the diameter d₁ of the piston 9 and by the diameter d₆of the plunge cut 17. The hydraulic fluid located in the first chamber13 exerts a force toward the left in FIG. 1 on the third annular faceA₃.

The fourth annular face A₄ is defined by a shoulder 51 of the piston 9in the region of the third chamber 25. The shoulder 51 is formed by thediameter d₂ and the diameter d₅ of the piston 9. The fourth annular faceA₄ always exerts a force acting counter to the opening direction on thepiston 9, since as already noted, the third chamber 25 is alwayssubjected to the feed pressure of the pump 31.

Since the first annular face A₁ is larger than the third annular face A₃and the fourth annular face A₄, the piston 9 moves to the right when thefirst chamber 13 is subjected to the feed pressure of the pump 31. Theannular piston 7 transmits the hydraulic force exerted upon it to thepiston 9, via the shoulder of the stepped center bore 19 of the annularpiston. The motion of the piston 9 to the right in FIG. 1 results in theopening of the gas exchange valve, not shown.

When the annular piston 7 and the piston 9 move to the right in terms ofFIG. 1, the volume of the second chamber 27 decreases. Since the secondswitching valve 33 is closed, the fluid positively displaced to theright from the second chamber 25 by the motion of the annular piston 7and piston 9 can flow only into the hydraulic reservoir 41. Thehydraulic fluid that flows into the hydraulic reservoir 41 moves thepiston 43 counter to the spring 45, until the piston 43 rests on thestop 47.

Once the piston 43 rests on the stop 47, no further hydraulic fluid canflow out of the second chamber 27 into the hydraulic reservoir 41, andthe result is that the volume of the second chamber 27 remains constant.This means nothing more than that the annular piston 7 can no longermove farther to the right. As a consequence, the hydraulic force thatmoves the piston 9 to the right decreases, since now only the hydraulicforce acting on the second annular face A₂ is available for opening tothe gas exchange valve, not shown.

The hydraulic forces described above, acting on the third annular faceA₃ and the fourth annular face A₄ and which seek to move the piston tothe left, that is, counter to the opening motion, remain unchanged. As aresult, the opening force acting on the gas exchange valve, not shown,decreases once the gas exchange valve has lifted from the valve seat,not shown.

In FIGS. 2a, 2 b and 2 c, various stages in the opening motion andclosing motion are shown, which are intended to illustrate what has beensaid above. In order not to overcomplicate the drawing, not all thereference numerals of FIG. 1 have been repeated in FIG. 2.

In FIG. 2a, the actuator is shown in a position in which the gasexchange valve is closed, and the full opening force is available.

In FIG. 2b, the state is shown in which the volume of the second chamber27 no longer decreases, since the pressure reservoir 41, not shown inFIG. 2, does not receive any further fluid. As a consequence, theannular piston 7 no longer moves. When the gas exchange valve is openedagain, the piston 9, with its diameter d₂, moves out of the steppedcenter bore 19 of the annular piston 7. From that position on, a directhydraulic communication exists between the first chamber 13 and thesecond chamber 27. This does not change the opening force at all.

In FIG. 2c, the hydraulic actuator is shown in a position in which thegas exchange valve is fully open, and the piston 9 has moved to theright out of the annular piston 7.

For closing the gas exchange valve, the piston 9 must be moved to theleft in terms of FIGS. 1 and 2. This is accomplished by closing thefirst switching valve 29 and opening the second switching valve 33. Thisposition of the switching valves 29 and 33 is shown in FIG. 1. Thehydraulic force exerted on the shoulder 51 of the piston 9 by the fluidlocated in the third chamber 25 at the feed pressure of the pump 31moves the piston 9 to the left. Hydraulic fluid is now pumped out of thefirst chamber 13 and second chamber 25 into the oil sump 35 via thecheck valve 39 and the second switching valve 33. In addition, thespring 45 of the hydraulic reservoir 41 is capable of lifting the piston43 from the stop 47 and moving the piston 43 onward into its outsetposition.

As soon as the piston 9 plunges with its diameter d₂ into the steppedbore 19 of the annular piston 7, the hydraulic fluid located in thefirst chamber 13 can no longer reach the oil sump 35 directly via thesecond chamber 27 and the line 37 but must instead flow into the oilsump 35 via the throttle 49. As a result, a certain overpressure buildsup in the first chamber 13 and the motion of the piston 9 is braked. Assoon as the annular piston 7 rests with its stepped inner bore 19 on thepiston 9, the annular piston 7 and the piston 9 move together. As aresult, a greater oil volume is pumped through the throttle 49, whichleads to a boosting of the braking action.

The position beyond which the desired braking of the gas exchange valveensues before the gas exchange valve strikes the valve seat, not shown,is dependent on the stroke of the reservoir piston 43 and is thus notdependent on the thermal expansion that the hydraulic actuator isexposed to. Nor do production tolerances of the actuator affect thisposition. As a result of the suitable choice of the diameters d₁ throughd₆, the ratios of the opening force upon liftoff of the gas exchangevalve from the valve seat and the reduced opening force upon furtheropening of the gas exchange valve and the closing force upon closure ofthe gas exchange valve can be adapted to one another, in order to attainan optimal operating performance of the hydraulic actuator.

In FIG. 3, the actuator of FIG. 1 is shown in fragmentary form, only tothe extent of interest below, with the housing 1, first chamber 13,second chamber 27 and third chamber 25, and with its hydraulicconnection to the hydraulic pump 31 with the first switching valve 29,embodied for instance as a 2/2-way magnet valve, and the hydrauliccommunication between the first chamber 13 and second chamber 27 via thethrottle 49. The hydraulic relief chamber or oil sump is identified, asbefore, by reference numeral 35, and the line connecting the secondchamber 27 with the second switching valve 33, embodied for instance asa 2/2-way magnet valve, is identified by reference numeral 37. Thehydraulic actuator has been modified to the extent that the device forlimiting the volumetric decrease of the second chamber 27, which in FIG.1 is embodied as a hydraulic spring reservoir 41, is now replaced with ashutoff valve 50, which is in communication with an opening in thesecond chamber 27, for instance being connected to the line 37, and inone switching position it closes the opening in the second chamber 27,or the connection to the line 37, while in its other switching positionit opens it so that fluid can flow out to the oil sump 35. The functionof this shutoff valve 50, represented only symbolically in FIG. 3, is,however, assigned to the second switching valve 33, which to enablefluid to flow out of the second chamber 27 is in the basic positionshown in FIG. 3 and which is switched over to its other switchingposition in order to block off the second chamber 27. The switchovervalve 33 furthermore maintains its function, already described inconjunction with FIG. 1, for the closure of the gas exchange valvewithout modification.

As described above, to open the gas exchange valve the first switchingvalve 29 must be opened. Fluid now flows at the feed pressure into thechamber 13, so that the piston 9 of the actuator is displaced togetherwith the annular piston 7 as shown in FIG. 2b. If at an arbitraryinstant during the displacement of the annular piston 7 the secondswitching valve is switched over to its blocking position, then fluidcannot flow out of the second chamber 27, and the annular piston 7 isblocked. The stroke of the annular piston 7 is accordingly defined bythe instant of switchover of the second switchover valve 33, which atthe onset of the opening motion of the actuator is open.

As described above, to close the gas exchange valve, the annular piston7 is displaced back again by the pressure in the third valve chamber 25,as soon as the first switching valve 29 is blocked again and the secondswitching valve 33 is opened again. In the process, the pressure in thefirst chamber 13 decreases via the throttle 49. After a stroke travel,the piston 9 strikes and carries the annular piston 7 along with it inits further stroke course. As a result, a high volumetric current and apronounced pressure increase in the first chamber 13 are caused, so thatthe piston 9 is braked sharply. The braking action begins at the instantwhen the annular piston 7 moves jointly with the piston 9, so that theinstant of onset of the braking operation is defined by the stroketravel of the annular piston 7, which is established in the openingprocess of the gas exchange valve. Thus by means of the instant ofswitchover of the second switching valve 33 into its blocking positionupon opening of the gas exchange valve, the instant of onset of thebraking event upon closure of the gas exchange valve can be defined.

The exemplary embodiment, shown in fragmentary form in FIG. 4, of theactuator with hydraulic connection is modified compared to FIG. 3 onlyto the extent that between the first chamber 13 in the housing 1 and thethrottle 49 in the connecting line to the second chamber 27 in thehousing 1, a flow-controlled valve 51 has been incorporated, which isembodied such that it is closable by the fluid flowing to the firstchamber 13. This flow-controlled valve 51 prevents fluid, in the initialphase for opening the gas exchange valve, in which phase both the firstswitching valve 29 and the second switching valve 33 are open, fromflowing directly from the first switching valve 29 out to the oil sump35 via the second switching valve 33; this is because the leakageflowing via the throttle 49 increases the energy requirement for valvecontrol, if it increases unacceptably. That is the case particularlywhenever the braking action upon the closure of the gas exchange valveis to be lowered moderately by means of a wider opening of the throttle49. If the first switching valve 29 is opened, then as a result of thefluid flowing from the hydraulic pump 31 into the first chamber 13, thevalve 51 is closed, and the communication with the throttle 49 is thusblocked. If the first switching valve 29 is closed, or in other wordshas been returned to the switching position shown in FIG. 4, then thevalve 51 opens, and the requisite communication for expelling the fluidfrom the first chamber 13 via the throttle 49 upon the closing of thegas exchange valve is reestablished.

The layout of the flow-controlled valve 51 is shown schematically inFIGS. 5 and 6; FIG. 5 shows the valve open, and FIG. 6 shows the valveclosed. The flow-controlled valve 51 has a housing 52, with a firstvalve connection 53 communicating with the chamber 13 of the actuator, asecond valve connection 54 connected to the throttle 49, and a thirdvalve connection 55 communicating with the outlet of the first switchingvalve 29. The first valve connection 53 communicates with a lower valvechamber 56, the third valve connection 55 communicates with an uppervalve chamber 57, and the second valve connection 54 communicates withan annular chamber 58 located between the lower and upper valve chambers56, 57. Between the lower valve chamber 56 and the annular chamber 58, avalve opening 60 surrounded by a valve seat 59 is embodied in thehousing 52. A guide sleeve 61 is inserted into the upper valve chamber57, and a valve member 62 embodied as a valve displacement piston isguided displaceably in this guide sleeve. The valve member 62 cooperateswith the valve seat 59 to close and open the valve opening 60, so thatthe annular chamber 58 is blocked off from the lower valve chamber 56when the valve member 62 is seated on the valve seat 59 (FIG. 6), andcommunicates with the lower valve chamber 56 when the valve member 62has lifted from the valve seat 59 (FIG. 5). A valve opening spring 63 isplaced in the lower valve chamber 56; it is embodied as a compressionspring and braced on one end on a shoulder 64 embodied in the lowervalve chamber 56 and on the other end on the valve member 62. The valveopening spring 63 presses the valve member 62 against a stop 65 embodiedin the guide sleeve 61.

The valve member 62 is provided with a central through opening 66, whichpermanently connects the upper valve chamber 57 with the lower valvechamber 56. The through opening 66 is embodied as a throttle, and forthat purpose its inner contour 67 has a design such that the fluidflowing from the upper valve chamber 57 to the lower valve chamber 56causes a pressure drop in the through opening 66. In the exemplaryembodiment of FIGS. 5 and 6, the through opening 66 has the form of adouble truncated cone for this purpose, in which two truncated cones areplaced on one another with their smaller bases.

If the first switching valve 29 is opened for the sake of opening thegas exchange valve, fluid flows from the outlet of the pump 31 throughthe through opening 66 in the valve member 62, and because of the innercontour 67, a pressure drop occurs between the upper and lower valvechambers 57, 56. Thus the pressure in the upper valve chamber 57 isgreater than in the lower valve chamber 56, and at the valve memberthere is a resultant displacement force, which counter to the springforce of the valve opening spring 63 seats the valve member 62 on thevalve seat 59 and thus closes the valve opening 60, as a result of whichthe communication with the throttle 49 is blocked.

If the first switching valve 29 is opened again, then no further fluidflows via the through opening 66. No pressure drop occurs at the innercontour 67, and so the pressures in the lower valve chamber 56 and inthe upper valve chamber 57 are equal. The force acting on the valvemember 62 is zero, and by means of the spring force of the valve openingspring 63, the valve member 62 is pressed against the stop 65 in theguide sleeve 61. The valve member 62 is thus lifted from the valve seat59, and the first chamber 13 of the actuator now communicates with thethrottle 49. Upon closure of the gas exchange valve, the fluid volumepositively displaced from the first chamber 13 as a result of thedisplacement motion of the pistons 9 and 7 can now flow out into the oilsump 35, via the throttle 49 and the opened second switching valve 33.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A hydraulic actuator for a gas exchange valve ofan internal combustion engine, comprising a cylinder bore (3), a piston(9), an annular piston (7), the piston (9) and the annular piston (7)being guided in the cylinder bore (3); the piston (9), annular piston(7) and cylinder bore (3) defining a first chamber (13) in the axialdirection whose volume increases when the actuator (1) opens the gasexchange valve (23), the annular piston (7) and the cylinder bore (3)defining a second chamber (27) in the axial direction whose volumedecreases when the actuator (1) opens the gas exchange valve (23); thepiston (9) and the cylinder bore (3) defining a third chamber (25) whosevolume decreases when the actuator (1) opens the gas exchange valve(23), and a device for limiting the volumetric decrease of the secondchamber (27).
 2. The actuator of claim 1, wherein the piston (9)comprises a plunge cut (17); wherein the annular piston (7) comprises astepped center bore (19) with one larger diameter (d₂) and one smallerdiameter (d₃); and wherein the annular piston (7) can be slipped by thelarger diameter (d₂) of the center bore (19) onto the piston (9).
 3. Theactuator of claim 2, wherein the diameters (d₁, d₂) of the piston (9) onboth sides of the plunge cut (17) are different; and wherein the annularpiston (7) can be slipped onto the larger diameter (d₂).
 4. The actuatorof claim 1, wherein the third (25) communicates directly, and the firstchamber (13) communicates via a first switching valve (29), with theoutlet of a pump (31) that generates feed pressure, and wherein thesecond chamber (27) communicates via a second switching valve (33) witha relief chamber (35) that receives fluid.
 5. The actuator of claim 4,wherein the device for limiting the volumetric decrease in the secondchamber (27) comprises a shutoff valve (50) which is in communicationwith an opening in the second chamber (27) and which in one switchingposition closes the opening and in its other switching position opens itto allow fluid to flow out, and wherein the shutoff valve is formed bythe second switching valve (33).
 6. The actuator of claim 1, wherein thedevice for limiting the volumetric decrease of the second chamber (27)comprises a pressure reservoir (41) that is in communication with thesecond chamber (27) and has a piston (43); and wherein the travel of thepiston is limitable.
 7. The actuator of claim 6, wherein the pressurereservoir (41) is a spring reservoir (45) or a gas reservoir.
 8. Theactuator of claim 7, wherein the travel of the piston (43) is limitableby means of a stop, in particular an adjustable stop (47).
 9. Theactuator of claim 6, wherein the travel of the piston (43) is limitableby means of a stop, in particular an adjustable stop (47).
 10. Theactuator of claim 1, wherein the device for limiting the volumetricdecrease in the second chamber (27) comprises a shutoff valve (50) whichis in communication with an opening in the second chamber (27) and whichin one switching position closes the opening and in its other switchingposition opens it to allow fluid to flow out.
 11. The actuator of claim1, wherein the first chamber (13) and the second chamber (27)communicate with one another via a throttle, in particular an adjustablethrottle (49).
 12. The actuator of claim 11, further comprising aflow-controlled valve (51) disposed between the first chamber (13) andthe throttle (49), the flow-controlled valve (51) being embodied suchthat it is normally open and can be closed by the fluid flowing to thefirst chamber (13).
 13. The actuator of claim 12, wherein theflow-controlled valve (51) comprises a housing (52) with a first valvechamber (56) in communication with the chamber (13), a second valvechamber (58) in communication with the throttle (49), a third valvechamber (57) in communication with the first switching valve (29), and avalve opening (60), disposed between the first and second valve chambers(56, 58) and surrounded by a valve seat (59), the flow-controlled valve(51) also comprising a valve member (62), which defines the third valvechamber (57) and is axially displaceable in the housing and whichcooperates with the valve seat (59) for closing and opening the valveopening (60), and a throttle opening (66), embodied in the valve member(62), which connects the first and third valve chambers (56, 57) withone another.
 14. The actuator of claim 13, wherein the throttle (49) isformed by the inner contour (67) of a central through opening (66) madein the valve member (62), which opening has an inner contour (67)designed such that the fluid flowing from the third valve chamber (57)into the first valve chamber (56) causes a pressure drop at the valvemember (62).
 15. The actuator of claim 14, wherein the inner contour(67) of the through opening (66) and the valve opening spring (63) areadapted to one another in such a way that the displacement force exertedon the valve member (62) as a result of the pressure difference isgreater than the contrary force of a valve opening spring (63).
 16. Theactuator of claim 15, wherein the through opening (66) has the form of adouble truncated cone, in which two coaxial truncated cones stand withtheir smaller bases on one another.
 17. The actuator of claim 14,wherein the through opening (66) has the form of a double truncatedcone, in which two coaxial truncated cones stand with their smallerbases on one another.
 18. The actuator of claim 13, wherein the throughopening (66) has the form of a double truncated cone, in which twocoaxial truncated cones stand with their smaller bases on one another.19. The actuator of claim 1, further comprising a check valve (39)between the second chamber (27) and the first chamber (13), the checkvalve (39) blocking the communication from the first chamber (13) to thesecond chamber (27).